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What do you mean by "Boost equivalent"? If you mean "a library for making the best use of C++ template metaprogramming features", then no, that wouldn't make sense for C. If you just mean "batteries included", then that's not really what Boost is anyway - a lot of obvious "batteries" (xml, other useful file formats, ...) aren't there whereas some very non-obvious choices are. Do you really want a library containing "you know, just a load of stuff" anyway, or do you have some specific needs?
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Steve314Oct 28 '11 at 3:49

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Boost is a project that's very close to WG21, the ISO C++ group. That's why so many boost features made it into C++11. There's no equivalent project associated with WG14, the ISO C group.
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MSaltersOct 28 '11 at 10:10

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@Steve314 In fact template metaprogramming is an equivalent of C's macro metaprogramming, so a Boost for C is in theory possible (-;
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mbqOct 28 '11 at 11:13

@mbq - last time I checked, C macros are not Turing complete. Maybe I'm wrong, though - e.g. can a macro recursively call itself?
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Steve314Oct 28 '11 at 14:22

@Steve314 With a little cheating with build or m4 yes; yet I doubt this is a crucial Boost's requirement.
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mbqOct 28 '11 at 15:42

3 Answers
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The Apache Portable Runtime (APR) is a supporting library for the
Apache web server. It provides a set of APIs that map to the
underlying operating system (OS). Where the OS doesn't support a
particular function, APR will provide an emulation. Thus programmers
can use the APR to make a program truly portable across platforms.

APR
originally formed a part of Apache HTTP Server, but the Apache
Software Foundation spun it off into a separate project. Other
applications can use it to achieve platform independence.

Another candidate would be Glib. It provides a bunch of functionality that is very useful, but not part of the C language or the C library. Stuff that is in the standard API in languages like Java or C#:

GLib is a cross-platform software utility library that began as part
of the GTK+ project. [...] GLib provides advanced data structures,
such as memory chunks, doubly and singly linked lists, hash tables,
dynamic strings and string utilities [...] GLib implements functions
that provide threads, thread programming and related facilities [...]
Some other features of GLib include: